Electrical codes typically require that ground fault circuit Interrupters (GFCIs) be installed in a plurality of locations. In homes, for example, GFCIs may be required to be installed in kitchens, bathrooms and garages. Many ground fault protectors are available commercially for use in 120 VAC and/or 240 VAC circuits. A typical home GFCI uses a differential current transformer that can detect a differential current as low as 5 milli-amperes on a pair of wires carrying 15 amperes of AC current. A detected difference between source and return currents typically indicates the presence of a ground fault. Building codes frequently specify, for example, that a 120 VAC circuit is to be opened in response to a detected ground fault current greater than 5 milli-amperes.
Although differential current detection devices generally are inexpensive and effective when used in AC circuits, they are not designed to detect faults in direct current (DC) circuits. Although electric circuit breakers are available with a DC ground fault trip capability, they are expensive and not sufficiently sensitive to detect the relatively low levels of fault current typically specified as ground fault trip thresholds when people are to be preferred.
One growing area of use for DC circuits is in supplying power for electric vehicles and hybrid electric vehicles. Automotive power sources can provide high voltages, e.g., between 400 and 800 volts DC, and it is desirable to provide ground fault detection in these systems. As previously discussed, however, the differential current detection devices which are currently available are expensive and inadequate for detecting differential current in DC circuits.